Yes you can ride the zip line instead of going on the ferry. It takes 90 seconds and costs 35 000 won whereas the ferry is free and included in your day trip

Agree with answer above. You have to have cash to give to the tour guide to pay for you. I think it is around 45000 KRW per person.

You can, if weather is ok. The proce for zipline is not included. You have to pay on spot